The most basic and important factor in the shadow is Yin Yang. The energy of
Yang in the air is called Yang Chi and the energy of Yin in the air is called
Yin Chi.

Daylight is getting longer and night time is getting shorter
from Winter Solstice to Summer Solstice. That means Yang Chi begins to
grow and Yin Chi begins to decline from Winter Solstice till Summer
Solstice.

Night time is getting longer and daylight is getting shorter from
Summer
Solstice to Winter Solstice. That means Yang Chi begins to decline and Yin
Chi begins to grow from Summer Solstice till Winter Solstice.

Yang Chi and Yin Chi are even on Vernal Equinox; Yin passes
its duty to Yang at this time. Yang Chi and Yin Chi are also even on Autumnal
Equinox; Yang leaves its duty to Yin at this time.

Based on the rise and fall relationship of Yin Yang,
Chinese divided a year into three Yang periods and three Yin periods. The
first Yang is cold. the second Yang is windy, and the third Yang is hot.
The first Yin is fiery, the second Yin is wet, and the third Yin is dry. These
concepts are applied to Chinese medicine later.

They used one to stand for the sky, which is Yang and used two to
stand for the ground, which is Yin. Therefore Yang has its symbol
and Yin has its symbol .
Later, Chinese divided the year into six Yang periods and six Yin
periods.

Six Yang periods are , ,
, ,
and .
The symbols tell that Yang Chi is rising. Six Yin periods are ,
, ,
,
and . All these symbols are part of the
I-Ching. The symbol, a series of six lines, is called Hexagram in I-Ching. When using
for calendar symbols, these hexagrams carry the weather
information.

Applying six Yin and Yang periods on the the life cycle of plant, Chinese
developed 12 Chinese pictographic characters as another counting system in the
following. These 12 characters were then borrowed into the Chinese calendar. Chinese call them 12 Branches.

Order

Chinese

Pronounce

Chinese Pictographic
Characters

Hexagram

Animal

Name

1 - Yang

Zǐ

(son)
Yang Chi begins to grow inside the ground. The seed begins to absorb water and
grow too.

Rat

2 - Yin

Chǒu

(twister) Yin Chi bends down into the ground. The spout of the seed twists
out in the ground.

Cow

3 - Yang

Yin

(develop) Yang Chi grows up from the ground. The Plant then completely
comes out from the cold soil.

Tiger

4 - Yin

Mǎo

(up) Sun rises. The Plant begins to spout the first new leaves, mixing with Yang Chi
and Yin Chi.